Regarding the signal wiring system of instruments

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In this world, the scariest thing is seriousness! Technically speaking, we must be serious! ——Ye Xiangdong – Author’s profile: Ye Xiangdong, male, graduated from the Department of Automation at Fushun Petroleum Institute in 1982. He is the deputy chief engineer at Sinopec Beijing Design Institute and Sinopec Engineering Construction Company, as well as a professor-level senior engineer. By Ye Xiangdong: With the development of the Internet, online knowledge has become increasingly complex, making it difficult to distinguish between truth and falsehood. Just how profound is the knowledge related to the wiring systems of instruments? This article is an editorial piece compiled from the Internet, but it contains many errors in its wording. By comparing it with the original source, this article aims to help readers properly understand the concepts of \"two-wire, three-wire, and four-wire\" systems for instrument signals. 1. The meaning of the three wiring types: Two-wire system: “Two wires serve both as power lines and as signal lines; it is generally used for 4–20mA signal transmission.” Identification: “Wiring system” refers to the number of wires used for powering the instrument and transmitting standard signals. Two-wire system uses two wires to supply power while transmitting a 4–20mA standard signal. A signal that does not meet this standard is not called two-wire. The two-wire supply capacity is: 4mA×24V – power consumption of the receiving instrument – line losses. Three-wire system: \"It refers to one wire serving as the positive power line, one wire as the positive signal line, and one wire as a common line for both the negative power line and the negative signal line.\" ； It is generally used for 1~5V signal transmission.” Identification: The three wires in a three-wire system are: one is the power wire (usually 24VDC+), one is the signal wire, and one is the common wire for both power and signal, which is used for the voltage reference point and the current loop. When the power supplied via a two-wire system is insufficient to power the instrument, a three-wire system is used, with a signal of 4–20 mA as the standard signal. A 1–5V signal only requires two wires. It is usually obtained by connecting a 250Ω resistor in series in a 4–20mA standard signal circuit. Four-wire system: “Refers to two wires for the power supply and two wires for the signal.” The power supply and the signal operate separately. That is, on the basis of a three-wire system, the signal line has its own ground, which is not shared with the power line. One’s own current is part of the total current, and since it is not at the same ground potential as the power supply, its value may be difficult to calculate compared to three-wire systems. ” Identification: An instrument for four-wire signals refers to one in which the power supply circuit and the signal circuit are independent of each other, separated by isolation circuits; it uses a standard 4–20 mA signal for isolation. Therefore, the signals must be isolated; they are usually ungrounded. The power supply can be either 24VDC or 220VAC. Meters powered by 220VAC must be four-wire systems. The four-wire system did not evolve from the three-wire system on the basis of \"separate grounds for signals and power,\" and the signal current is not part of the total current. The supply current is the ratio of the instrument’s rated power to the power supply. 2. Similarities and differences among the three wiring systems. Original text: Advantages and disadvantages of two-wire, three-wire, and four-wire systems. Note: The choice of wiring system is determined by the power supply requirements of the instrument, so it’s not possible to simply compare their advantages and disadvantages. The advantage of the two-wire system is its simple wiring; it is only suitable for primary sensors with low power, such as voltage transformers, differential pressure transducers, temperature sensors, capacitive level gauges, radio frequency admittance meters, electromagnetic flowmeters, vortex flowmeters, etc. The power supply for the sensor itself comes from a two-wire system, which inevitably affects its load-carrying capacity. Identification: The 2-wire 4–20 mA signal is an international standard signaling system, used in electronic unitized instruments, various low-power transmitters, sensors, etc. Since instruments typically use a standard supply voltage of 24VDC, the power supply capacity is somewhat limited. The advantage of the three-limit method is that the thermal resistor is connected using a three-wire configuration, which helps to eliminate measurement errors caused by the resistance of the connecting wires. This is because the circuit for measuring the thermistor is an unbalanced bridge. As one of the arm resistances in a bridge, the thermistor’s connecting wire (from the thermistor to the control room) also constitutes part of the arm resistance. This component’s resistance is unknown and changes with temperature, resulting in measurement errors. A three-wire system is adopted, with one wire connected to the power supply terminal of the bridge, and the other two wires connected respectively to the bridge arm where the thermal resistor is located and to the adjacent bridge arm; this eliminates the measurement errors caused by the resistance of the wiring. Identification: Thermal resistors can be connected using two-wire, three-wire, or four-wire configurations. However, these are not standard signals; they differ from the standard signals associated with the three types of wiring schemes and must not be confused with one another! “There are errors in the usage of words and phrases such as “is necessary”, “dispel”, “measure”, “connect”, “partial resistance”, “distinguish and connect to”, “such as”! It should be: inevitably, reduce, measure, connect, partial resistance, connect separately, achieve, and so on. The purpose of the three-wire system is to supply power to the instrument when the power provided by the two-wire system is insufficient for that purpose. The advantage of the four-wire configuration is that since the power supply and signal lines are separated, there is no power-related relationship between the device’s power output and the signal. This makes it suitable for high-power sensors, such as ultrasonic sensors. Since these sensors require a relatively high transmission power in order to enhance their interference resistance, it is advisable to opt for a four-wire configuration when selecting such products; two-wire configurations generally offer weaker interference resistance. Therefore, they cannot be designed with only two wires—they must have four wires in total: two for the operating power supply and two for the output signals. Identification: “Separating power supply and power” should be “Separating power supply and signals”. “The statement “There is no power-related relationship between the power and the signal of this device” is inaccurate! The instrument’s rated power is the sum of the load power and the power consumed by the instrument itself; the signal load power is included in the instrument’s rated power consumption. “The statement “In order to enhance anti-interference capability, the transmission power is set very high” is also inaccurate. Ultrasonic instruments consume a high amount of power in order to achieve a strong ultrasonic detection signal, so that the detection unit receives sufficient signal strength and thus obtains a better detection result. “\"Anti-interference\" is a broad term that can refer to electromagnetic interference, interference from environmental conditions, interference from similar stray signals, and so on. Anti-interference measures are complex comprehensive approaches, and they cannot be achieved merely by increasing power. ““2 power supplies, 2 outputs” should mean two power wires and two signal wires. 3. A wire-type system has only one wiring method. Original text: Within the family of sensors, in addition to two-wire sensors, there are also three-wire and four-wire sensors ； Not only sensors, but also devices such as transmitters, level gauges, and flow meters have different wiring methods... Identification: Wiring system and wiring method are two distinct things; each wiring system has only one corresponding wiring method. 4. The wiring configuration has nothing to do with interference. Original text: What’s the difference between two-wire and three-wire sensors? Compared to three-wire systems (one positive power line, two signal lines, one of which shares the GND) and four-wire systems (two positive and negative power lines, two signal lines), the advantages of the two-wire system are as follows: Identification: In a three-wire system, there is one power line (usually 24VDC+), one signal line, and one common line for both power and signals. (1) It is not affected by parasitic thermocouples, resistance drops along the wires, or temperature drift, and can use much cheaper, thinner wires ； It can save a large amount of cable and installation costs ； Identification: Completely wrong! The advantage of a two-wire system is that it uses only two wires for power supply and signal transmission, thus saving wires. What is a “parasitic thermocouple”? Regardless of the number of wires, there is line resistance, and a two-wire system does not reduce the impact of this resistance. “\"Temperature drift\" is a parameter of the sensor, not a parameter of the wiring. (2) When the output resistance of the current source is large enough, the voltage induced in the wire loop through magnetic field coupling has no significant effect, as the current caused by interference sources is minimal; generally, twisted pairs can be used to reduce such interference ； For two-wire and three-wire systems, shielded wires must be used, and the shielding layer of these wires should be properly grounded ； Identification: The meaning of this sentence is unclear. Even if the internal resistance of the current source is high, since the signal load is fixed (usually 250Ω), interference signals are sufficient to affect the measured signal; the effect will be present regardless of the current in the wires. Otherwise, there would be no need to take measures to reduce interference. Electromagnetic induction generates current, not voltage. Identification: Whether to use shielded wires has nothing to do with two-wire or three-wire systems; it depends on the interference environment, and it is not \"mandatory\" to use shielded wires. Generally, standard signal wires do not use twisted pairs when there is no electromagnetic field along the wire path. “The phrase “proper grounding” is not accurate; what exactly constitutes proper grounding? (3) Capacitive interference can cause errors related to the receiver resistance. For 4–20 mA two-wire loops, the receiver resistance is typically 250 Ω (with a sampled Uout of 1–5 V). This resistance value is too low to produce any significant errors. Therefore, the permissible length of the wiring can be longer than that in voltage telemetry systems ； Identification: The interference causes signal errors, not errors in the resistance of the “receiver”. The signal error caused by interference currents on a 250Ω signal resistor cannot be overlooked; therefore, measures must be taken to reduce such interference currents. What is the wire length that is longer and farther than that of the “Voltage Telemetry System”? (4) Each individual reading or recording device can be connected to different channels with wires of varying lengths, without any difference in accuracy resulting from these length differences; this enables decentralized data collection. The advantage of decentralized collection is that it allows for data collection at various locations while maintaining centralized control. Identification: This passage has nothing to do with wiring systems, let alone being a characteristic of two-wire systems. (5) Using 4mA as the zero level makes it very convenient to determine whether there is an open circuit, a short circuit, or sensor damage (0mA condition). Identification: The 4 mA value in the 4–20 mA standard signal is not the \"zero level\"; rather, it represents the starting point of the physical signal being measured, that is, the zero point of the measurement signal. It is easier to distinguish the zero signal, the starting zero point, and line faults compared to the original 0mA. (6) It is very easy to add one or two surge and lightning protection components at the two-line output ports, which helps ensure safety against lightning and explosions. Identification: The installation of surge protectors is independent of the wiring system; there are two-wire, three-wire, and four-wire versions, as well as specifications tailored to different protection requirements. Lightning protection devices have nothing to do with explosion prevention! 5. The wiring scheme for signal lines differs from that of thermal resistors. Identification: The wiring scheme for signal lines refers to the number of wires used for powering the instrument and transmitting standard signals. The number of leads of a thermal resistor refers to the wiring method. The two are not the same thing; they cannot be confused. The signal wiring standard for instruments is fixed and cannot be changed. The number of leads for the thermal resistor can be chosen as needed. In the two-wire system, there is no compensation for line resistance; the wiring is simple, but it introduces additional errors due to lead resistance. Therefore, thermoresistors with Class A accuracy are not suitable, and the leads and wires should not be too long when in use. Identification: The correct statement is that the two-wire wiring method for thermal resistors lacks line balance compensation, which can lead to measurement errors due to line resistance. The three-wire system features wire resistance compensation, which can eliminate the impact of lead resistance, resulting in a higher measurement accuracy than the two-wire system. As a process detection element, it is the most widely used. Identification: The correct statement is that the three-wire wiring method for thermal resistors includes balance wires, which are used for compensation in order to eliminate measurement errors caused by line resistance. The three-wire wiring method is not a process monitoring element; the meaning of the latter sentence is unclear. The change in resistance of the two-wire sensor, together with the resistance of the connecting wires, determines the sensor’s output value. The additional error caused by the wire resistance results in a measured value that is higher than the actual value. This type of sensor is suitable for applications where high measurement accuracy is not required; moreover, the length of the wires should not be excessive. Identification: The correct term is two-wire wiring for thermal resistors. Three-wire system: It requires that the cross-sectional areas and lengths of the three wires used be identical. The circuit for measuring platinum resistors is usually an unbalanced bridge, with the platinum resistor serving as one of the resistor arms of the bridge. One wire is connected to the power supply terminal of the bridge, while the other two wires are connected respectively to the arm where the platinum resistor is located and to the adjacent arm. When the bridge is in balance, any changes in the resistance of the wires have no effect on the measurement results, thereby eliminating the measurement errors caused by the resistance of the wires. However, it must be an equilateral bridge; otherwise, it is not possible to completely eliminate the influence of wire resistance. Using a three-wire system **reduces the additional errors caused by wire resistance; therefore, the three-wire connection method is commonly used in industrial applications.** Identification: The correct statement is that in the three-wire connection for thermal resistors, three wires with the same cross-sectional area and length are used, which are connected properly to the two arms of the measuring bridge as well as to the power supply terminals, in order to eliminate the effect of wire resistance. Since there are instructions for both the wiring and installation of the instrument, and internal wiring is not involved, simply follow the instructions to wire it correctly. 6. Differences in the wiring methods of thermal resistors. Original text: What are the differences between two-wire thermal resistors, three-wire thermal resistors, and four-wire thermal resistors? Identification: Thermal resistors do not have signal wires; there are only differences in the wiring methods. The same thermistor can be connected using two-wire, three-wire, four-wire, and other wiring methods. In the two-wire system, there is no compensation for line resistance; the wiring is simple, but it introduces additional errors due to lead resistance. Therefore, thermoresistors with Class A accuracy are not suitable, and the leads and wires should not be too long when in use. The three-wire system includes wire resistance compensation, which can eliminate the influence of lead resistance, resulting in a higher measurement accuracy than the two-wire system. As a process detection element, it is the most widely used. The four-wire system refers to the method in which two wires are connected to each end of the thermal resistor; these two wires supply a constant current I to the thermal resistor, converting R into a voltage signal U, which is then transmitted to the PLC via the other two wires. This type of lead configuration can completely eliminate the influence of lead resistance, but it is costly and is mainly used for high-precision temperature measurement. Identification: The accurate term should be: two-wire, three-wire, and four-wire wiring methods for thermal resistors. “As a process detection element, this wiring method should be used. 7. Two-wire transmitters: What is the difference between two-wire transmitters and three-wire serta four-wire transmitters? To implement a two-wire transmitter, the following conditions must be met simultaneously: 1. V ≤ Emin – Imax·RLmax. The output voltage V of the transmitter is equal to the specified minimum supply voltage minus the voltage drop caused by the current across the load resistance and the transmission wire resistance. 2. I≤Imin: The normal operating current I of the transmitter must be less than or equal to the output current of the transmitter. Clarification: The correct statement is that the normal operating current of a transmitter is its output signal current. This is an inherent characteristic, not a condition. 3. P＜Imin(Emin-IminRLmax): The minimum power consumption P of the transmitter must not exceed the value given in this formula; it is usually less than 90 mW. In the formula: Emin = minimum supply voltage; for most instruments, Emin = 24(1–5%) = 22.8 V, with 5% representing the allowable negative variation for a 24 V supply ； Imax=20mA ； Imin=4mA ； RLmax=250Ω+resistance of the transmission wire. If the transmitter meets the above three conditions in its design, two-wire transmission can be achieved. The so-called two-wire system means that the power supply and the load are connected in series, sharing a common point; the signal communication as well as the power supply between the field transmitter and the instruments in the control room are accomplished using just two wires, which serve both as power lines and signal lines. In two-wire transmitters, the signal starts at 4 mA.DC, which provides a static operating current for the transmitter. Meanwhile, the electrical zero point of the instrument is also 4 mA.DC, and it does not coincide with the mechanical zero point. This \"dynamic zero point\" facilitates the detection of faults such as power outages and broken connections. Moreover, the two-wire system also facilitates the use of safety barriers, contributing to safety and explosion prevention. Identification: There are only two conditions, 1 and 3; if it is an intrinsically safe system, the voltage drop across the safety barrier must also be taken into account. The definition of a two-wire system is not that the power supply and the load are connected in series with a common point, but rather that two wires are used to supply power while simultaneously transmitting a 4–20mA standard signal; these two wires serve both as power lines and as signal lines. ““Signal liaison” should be signal transmission ； “The “instrument electrical zero point” should be the starting point of the signal. The last sentence: “Two-wire systems also facilitate the use of safety barriers” is incorrect! For two-wire, three-wire, and four-wire systems, there are corresponding safety barriers available for use in intrinsically safe systems; there is no such thing as being \"more convenient\" among them. 8. Three-wire transmitters: Some instrument manufacturers, in order to reduce the size and weight of transmitters, improve their resistance to interference, and simplify wiring, have changed the power supply for these transmitters from 220V AC to low-voltage DC power. If the power is supplied from a 24V DC power supply unit, the low-voltage power supply facilitates the use of a common negative wire, which leads to the development of three-wire transmitter products. Identification: Three-wire instruments are not derived from four-wire instruments powered by 220VAC, nor are they designed to reduce the size and weight of the transmitter; rather, they are designed based on the power supply requirements of the instrument. The purpose of using 24VDC power is to reduce instruments powered by 220VAC, thereby minimizing the use of non-safe voltage levels. Three-wire instruments are not therefore more resistant to interference than four-wire instruments. As can be seen from the above description, due to the different working principles and structures of various transmitters, different products have been developed, which in turn determines the two-wire, three-wire, and four-wire wiring configurations for transmitters. For users, when making a selection, they should take into comprehensive consideration the actual conditions of their own organization, such as the standardization of signaling systems, explosion-proof requirements, specifications for receiving equipment, and investment considerations. Identification: When selecting instruments, there is no need to consider wiring systems; it is sufficient to connect the wires correctly according to the wiring system of the chosen instrument. 9. Four-wire transmitters: Due to the widespread use of the 4-20mA.DC (1-5V.DC) signal standard, it is necessary to have a unified signal standard in control systems to facilitate connections. As a result, certain instruments that are not electrically powered, such as those used for on-line analysis, measurement of mechanical quantities, and electrical quantities, are required to be able to output signals in the 4-20mA.DC format. However, due to complex conversion circuits and high power consumption, it is difficult to meet all three requirements; hence, a two-wire system cannot be used. In such cases, a four-wire transmitter with an external power supply is required to generate 4-20mA.DC output signals. Identification: The transmission of instrument signals must be in the same standard; otherwise, the signal-receiving instrument will not be able to receive or identify them. It is not a matter of ease of connection. Electric unit combination instruments also include four-wire types, such as the DDZⅡ type instruments. Most four-wire instruments are those powered by 220VAC, or instruments that require a high power supply. As shown in Figure 2, four-wire transmitters are generally powered by 220V AC, although some are also powered by 24V DC. The output signal can be 4-20mA.DC with a load resistance of 250Ω, or 0-10mA.DC with a load resistance of 0-1.5KΩ ； Some also have mA and mV signals, but the load resistance or input resistance varies depending on the type of output circuit. Identification: Since the 1980s, four-wire instruments have all adopted the 4–20 mA standard signal, replacing the 0–10 mA DC signal system. Apart from the thermocouple signal being a self-generated mV signal, there are no standard mV-type signals. The load resistance of the output signal instrument is the input resistance of the signal receiving instrument, which is determined by the input circuit of the signal receiving instrument and the requirements of signal recognition. Most are 250Ω; generally, they range from 50 to 750Ω, and it is impossible for them to be zero. 10. Wiring of instruments and signal isolation: It should be noted that the 4-20mA.DC signals output by three-wire and four-wire transmitters, due to the differences in their output circuit principles and structure compared to two-wire transmitters, raise the question of whether their negative terminals can be connected to the negative wire of a 24V power supply, and whether common grounding is possible. This is something that needs to be taken into account; if necessary, isolation measures such as distributors or safety barriers can be employed to enable sharing of power and grounding with other instruments while avoiding the generation of additional interference. Identification: The instrument wiring should be carried out in accordance with the instruction manual; it cannot be done arbitrarily. There is also no need to worry about issues such as “common ground”. The grounding of instruments has a specific meaning; it is not related to the \"grounding\" of the instrument’s power supply and signals, nor to wiring issues. The isolation measure involves the use of isolators, not distribution units and safety barriers. Isolated safety barriers provide isolation, but their main purpose is to form a intrinsically safe system. The use of isolation measures does not mean sharing the same power supply or ground with other instruments; this is another important topic: the application of isolators. 11. The signal wiring system cannot be changed. Original text: Since there are two-wire, three-wire, and four-wire systems, can the wiring methods be exchanged among these? Let’s take a look together at how to change from two lines to four lines, and from four lines back to two lines As can be seen from the above, various wiring types of transmitters exist, and there is always a reason for that; otherwise, there wouldn’t be so many different wiring types. It is difficult for users to change the wiring type, and it also has little practical significance. Identification: Once the instrument design is finalized into a product, the wiring specifications become fixed and cannot be changed. Different wiring systems have different uses; this is something taken into account during the design of the instrument itself, rather than something that relates to how the instrument is used. To use the instrument, simply connect the wires according to the instructions. If a four-wire transmitter that transmits signals in the range of 0–10 mA.DC is to be converted to a two-wire system, the first problem that arises is that its starting current is zero. With no current flowing, the electronic amplifier of the transmitter cannot establish an operating point, which makes it difficult for the transmitter to function properly. If a DC power supply is used and the instrument’s original constant current characteristic is to be maintained, when the load resistance of the transmitter is between 0–1.5 KΩ and the feedback coil resistance connected in series is around 2 KΩ, the voltage drops across these two components will exceed 24 V when the output current is 10 mA. In other words, it is impossible to maintain a constant current characteristic when using a 24 V DC power supply with a load resistance of 0–1.5 KΩ; therefore, two-wire transmission is not feasible in such a scenario. Identification: During the mass production of the instrument, the signal specifications are fixed and cannot be changed by the user. Changing the wiring configuration of existing instruments is meaningless and impossible. Any attempt to change is futile.